Perspectives of microwave quantum key distribution in the open air
Florian Fesquet, Fabian Kronowetter, Michael Renger, Qiming Chen, Kedar Honasoge, Oscar Gargiulo, Yuki Nojiri, Achim Marx, Frank Deppe, R. Gross, Kirill G. Fedorov
One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this paper, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure with communication at room temperature up to distances of around 200 m, limited by the total coupled noise photon number. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelengths and imperfect weather conditions.
One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this paper, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure with communication at room temperature up to distances of around 200 m, limited by the total coupled noise photon number. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelengths and imperfect weather conditions.
Security of modern classical data encryption often relies on computationally hard problems, which can be trivialized with the advent of quantum computers. A potential remedy for this is quantum communication which takes advantage of the laws of quantum physics to provide secure exchange of information. Here, quantum key distribution (QKD) represents a powerful tool, allowing for unconditionally secure quantum communication between remote parties. At the same time, microwave quantum communication is set to play an important role in future quantum networks because of its natural frequency compatibility with superconducting quantum processors and modern near-distance communication standards. To this end, we present an experimental realization of a continuous-variable QKD protocol based on propagating displaced squeezed microwave states. We use superconducting parametric devices for generation and single-shot quadrature detection of these states. We demonstrate unconditional security in our experimental microwave QKD setting. The security performance is shown to be improved by adding finite trusted noise on the preparation side. Our results indicate feasibility of secure microwave quantum communication with the currently available technology in both open-air (up to ~ 80 m) and cryogenic (over 1000 m) conditions. Microwave-based quantum key distribution would allow a secure exchange of information in superconducting local area networks and, potentially beyond, in open-air conditions. Here, the authors provide a proof-of-principle implementation within a cryogenic environment, based on propagating displaced squeezed microwave states.
Continuous-variable quantum key distribution (CV-QKD) enables the efficient distribution of unconditionally secure keys for both communicating parties. Despite significant advancements in optical-based CV-QKD, a persistent challenge is the limited resilience of optical quantum signals to atmospheric noise in free-space channels. This susceptibility leads to considerable degradation in the performance of optical CV-QKD systems, particularly under adverse climatic conditions. Meanwhile, recent advancements in superconducting quantum computing have driven the demand for quantum communication between spatially separated superconducting processors operating at microwave frequencies. To solve the problems, we propose a Gaussian-modulated coherent-state protocol operating at microwave frequencies, utilizing traveling-wave parametric amplifiers to generate entangled states. Our numerical analysis demonstrates that CV-QKD at microwave frequencies enables secure communication in a free-space channel at room temperature. Furthermore, we show that this protocol outperforms the THz frequency protocol over a distance of 20 m and exhibits stronger immunity to interference. Finally, we consider the impact of microwave antenna gain on the protocol's performance, highlighting the decisive role of antenna gain in determining the maximum transmission distance in practical application scenarios.
Microwave technology plays a central role in current wireless communications, including mobile communication and local area networks. The microwave range shows relevant advantages with respect to other frequencies in open-air transmission, such as low absorption losses and low-energy consumption, and in addition, it is the natural working frequency in superconducting quantum technologies. Entanglement distribution between separate parties is at the core of secure quantum communications. Therefore, understanding its limitations in realistic open-air settings, especially in the rather unexplored microwave regime, is crucial for transforming microwave quantum communications into a mainstream technology. Here, we investigate the feasibility of an open-air entanglement distribution scheme with microwave two-mode squeezed states. First, we study the reach of direct entanglement transmission in open air, obtaining a maximum distance of approximately 500 m with parameters feasible for state-of-the-art experiments. Subsequently, we adapt entanglement distillation and entanglement swapping protocols to microwave technology in order to reduce the environment-induced entanglement degradation. The employed entanglement distillation helps to increase quantum correlations in the short-distance low-squeezing regime by up to $46\mathrm{%}$, and the reach of entanglement increases by $14\mathrm{%}$ with entanglement swapping. Importantly, we compute the fidelity of a continuous-variable quantum teleportation protocol using open-air-distributed entanglement as a resource. Finally, we adapt this machinery to explore the limitations of quantum communication between satellites, where the impact of thermal noise is substantially reduced and diffraction losses are dominant.
Quantum communication exploits non-classical correlations to achieve efficient and unconditionally secure exchange of information. In particular, the quantum teleportation protocol allows for a deterministic and secure transfer of unknown quantum states by using pre-shared quantum entanglement and classical feedforward communication. Quantum teleportation in the microwave regime provides an important tool for high-fidelity remote quantum operations, enabling distributed quantum computing with superconducting circuits and potentially facilitating short-range, open-air microwave quantum communication. In this context, we consider practical application scenarios for the microwave analog quantum teleportation protocol based on continuous-variable states. We theoretically analyze the effect of feedforward losses and noise on teleportation fidelities of coherent states and show that these imperfections can be fully corrected by an appropriate feedforward gain. Furthermore, we consider quantum teleportation with finite-size codebooks and derive modified no-cloning thresholds as a function of the codebook configuration. Finally, we analyze the security of quantum teleportation under public channel attacks and demonstrate that the corresponding secure fidelity thresholds may drastically differ from the conventional no-cloning values. Our results contribute to the general development of quantum communication protocols and, in particular, illustrate the feasibility of using quantum teleportation in realistic microwave networks for robust and unconditionally secure communication.
Abstract Quantum teleportation has achieved significant progress in the optical frequency domain but faces persistent challenges, including high transmission losses and limited transmittance. In contrast, the microwave frequency domain offers distinct advantages, such as reduced absorption losses, lower energy consumption, and compatibility with superconducting quantum technologies. To overcome the limitations of optical quantum teleportation, this study presents a microwave-based continuous-variable quantum teleportation (CVQT) scheme designed for open-air environments at room temperature. The proposed approach employs microwave two-mode squeezed states, with entanglement enhanced through quantum scissors operations to mitigate environmental degradation. Numerical simulations demonstrate that the maximum transmission distance of microwave entangled states for CVQT is about 200 m at room temperature. The results also indicate that the microwave CVQT scheme achieves lower attenuation and higher fidelity over short distances compared to optical frequency CVQT under similar weather conditions. Moreover, microwave quantum states exhibit reduced sensitivity to weather-induced variations, highlighting their robustness and practicality for open-air quantum communication applications.
Abstract Continuous variable quantum key distribution (CVQKD) can achieve the secure communication between two distant legitimate users. The microwave CVQKD exhibits stronger robustness than the optical CVQKD in transmitting quantum signals and accordingly has broad application prospects in quantum networks. Although the microwave CVQKD does not rely on the traditional optical fibers to transmit quantum signals, its practical security is still vulnerable to saturation attacks. In view of this security vulnerability, we generalize saturation attacks to microwave CVQKD and clarify the impact of saturation attacks on the parameter estimation of microwave systems. Two saturation attack schemes are further compared, one based on displacement control alone and the other on joint displacement and gain control. To mitigate the negative impacts caused by this attack, we propose a countermeasure based on the quantum k nearest neighbor algorithm, which can detect saturation attacks by measuring unknown quantum states. In addition, the influence of microwave antennas on the security of the communication system is assessed, confirming that antenna gain plays a key role in determining both the secret key rate and the maximum transmission distance.